Two closely related strains of rats with a differential susceptibility for plaque production have been developed from the Wistar-Kyoto Rat Strain. Plaque-susceptible rats develop large plaque deposits after two weeks on a soft diet, while plaque-resistant rats do not develop plaque at any time. The immunological status of these two strains has been investigated. The functional ability of both T and B lymphocytes was identical in these strains. Serum immunoglobulin and salivary IgA levels were also identical, in rats on a hard diet. However, plaque-susceptible rats exhibit elevated serum IgM after a prolonged period on a soft diet which produces gingival plaque deposits. This elevation is due to the plaque since it is not present in plaque-susceptible rats fed a hard diet which remain plaque-free. These findings suggest that the abnormal accumulation of plaque in these rats is not due to an immunological abnormality.